A common procedure in ophthalmology to remove cataracts is to use phacoemulcification and suction to remove the affected lens from the eye. This procedure is shown and described, for instance, in Cupler, U.S. Pat. No. 4,002,169, in which access to the lens is achieved through the sclera or limbus, then through the opening in the iris, and finally through the anterior capsular membrane. Although, as shown in Cupler, it may be possible to gain access to the lens and remove it by piercing the anterior capsular membrane, it is more common to remove a circular piece of the membrane by a "capsulotomy" procedure, for example by tearing out a portion of the membrane with a needle and forceps. Alternatively, it is known to free the desired membrane portion with a surgical knife. In this type of procedure, it is essential to maintain the cutting edge of the knife so that it is oriented in the proper direction during the entire circular cut. This may be difficult if the cutting instrument is introduced through the single incision in the sclera that is used for the other steps in the cataract removal.
One example of a surgical knife appropriate for a capsulotomy is disclosed in Pazandak, U.S. Pat. No. 4,708,138, in which a knife blade is attached to an extension handle at a swivel point, the cutting portion of the blade being off center from the swivel point. The knife is directed by sideways motion of the end of the handle removed from the blade attachment point, and the shape and method of attachment of the cutting blade to the handle cause the blade to orient itself with the cutting edge forward, in the direction of movement of the blade, as long as movement of the blade is continuous.